Dairy Chemistry And Biochemistry

260 DAIRY CHEMISTRY AND BIOCHEMISTRY

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5.8 Changes in milk salts equilibria induced by various treatments

The equilibria between the soluble and colloidal salts of milk are influenced
by many factors, the more important of which are discussed below, and
which consequently modify the processing properties of milk.
Milk serum is supersaturated with calcium phosphate, the excess being
present in the colloidal phase, as described above. The balance between
the colloidal and soluble phases may be upset by various factors, includ-
ing changes in temperature, dilution or concentration, addition of acid,
alkali or salts. The solubility product for secondary calcium phosphate,
[Ca2+][HPO:-] is about 1.5 x or pK, = 4.85.

5.8.1
Acidification of milk is accompanied by a progressive solubilization of
colloidal calcium phosphate and other colloidal salts from casein. Solubiliz-
ation is complete below about pH 4.9 (Figure 5.11).
Addition of alkali has the opposite effect, and at about pH 11 almost all
the soluble calcium phosphate occurs in the colloidal phase. These changes
are not reversible on subsequent dialysis against untreated milk.

Addition of acid or alkali

b)

(^3) - a
v1^0
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5 5.5 6 6.5 7
PH
Figure 5.11 Effect of pH on the distribution of calcium (O), inorganic phosphorus (O),
magnesium (0) and citrate (A) between the colloidal and soluble phases in bovine milk.